The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
“High solids” is a designation given in the industrial and automotive coatings fields to coating compositions or paints that are solventborne compositions and have a higher nonvolatile content, such as a nonvolatile content of more than about 40 weight percent (wt. %). The nonvolatile content is determined in accordance with ASTM Test Method D2369, in which the test sample is heated at 110° C. (230° F.) for 60 minutes. Coatings manufacturers have worked to reduce solvent content of industrial and automotive coating compositions for decades and have over the years developed various higher solids technologies. The solids content that can be achieved for a particular coating composition depends to a certain extent on the type of coating it is and the properties it must have on the substrate. It is generally understood that, while very low molecular weight resins allow less solvent to be added, one may not be able to achieve the necessary application and cured coating properties using such low molecular weight resins. As another example, one is generally able to make a sprayable clearcoat coating composition with a higher solids content than a sprayable pigmented coating composition. The dispersed pigment tends to increase viscosity of the coating so that more solvent must be added to obtain a suitable spray viscosity (that is, a suitable viscosity for application by spraying the coating onto the substrate).
The color and appearance of the coating can be of primary importance, as is true, for example, for automotive topcoat coatings. The color for these topcoats are provided by monocoat topcoat coatings, which are a single-layer topcoat, or basecoat coatings, which are used as the color layer under a clearcoat coating layer in a composite two-layer topcoat. Special effect colors, e.g. metallic and pearlescent colors and coatings with color-variable pigments, present an added challenge for these topcoat coatings. special effect flake pigments. Special effect pigments are those that can produce a gonioapparent effect in a coating layer. For example, the American Society of Testing Methods (ASTM) document F284 defines metallic as “pertaining to the appearance of a gonioapparent material containing metal flake.” Metallic basecoat colors may be produced using metallic flake pigments like aluminum flake pigments including colored aluminum flake pigment, copper flake pigments, zinc flake pigments, stainless steel flake pigments, and bronze flake pigments and/or using pearlescent flake pigments including treated micas like titanium dioxide-coated mica pigments and iron oxide-coated mica pigments to give the coatings a different appearance or color when viewed at different angles. Rheology control is needed during application of these coating compositions to allow the flakes to orient parallel to the face of the film for optimum gonioapparent effect. The flake pigments that produce metallic and pearlescent colors and colors that vary with viewing angle must, during drying of the applied coating layer, achieve an orientation substantially parallel to the substrate to provide the optimum desired metallic, pearlescent, or color-variable effect. High solids coating compositions with these pigments, grouped generally as “metallic” coating compositions, have not provided the outstanding difference in brightness between face (viewed head-on) and flop (viewed at an oblique angle) that can be achieved for low solids, high-solvent-content coatings. Obtaining proper rheology control during application and cure of pigmented high solids topcoats, especially when using high solids metallic topcoat compositions, while continuing to meet the stringent performance requirements for such coatings remains a demanding task.
Unpigmented clearcoat topcoat coatings require some kind of rheology control agent to allow a extremely high degree of surface smoothness to achieve a high distinctness of image (DOI). Clearcoat and monocoat topcoat coating layers are generally relatively thick, typically between 1.5 and 3 mils (about 38 to about 76 microns) thick for both appearance and protection. In coating automotive vehicle bodies, the topcoat is applied to both horizontal and vertical surfaces. Manufacturing economy constraints require this relatively thick clearcoat or monocoat topcoat layer be applied in a minimum of time and manufacturing floor space; accordingly, the clearcoat or monocoat coating composition is applied thickly onto the substrate, leaving in the coating layer a significant amount of solvent that must be evaporated before bake, during a “flash” period of solvent evaporation, and during bake of the topcoat. While there is less of a problem on horizontal surfaces with applying a rather thick coating layer leaving significant solvent content in the layer, on vertical surfaces a topcoat layer with still significant solvent content may flow too much, causing sags to develop in the coating layer. Sagging may also occur in other areas where the substrate is not flat horizontally, for example along character lines, gutters, or channels of an automotive vehicle body. Thus, rheology control is important for this reason as well.